Binder for electrode of lithium ion secondary battery

ABSTRACT

A binder for electrode of lithium ion secondary battery, comprised of a copolymer composed of 15 to 80 weight % of units from ethylenically unsaturated monomer (A) whose homopolymerization yields a polymer soluble in N-methylpyrrolidone (NMP) and 20 to 85 weight % of units from ethylenically unsaturated monomer (B) whose homopolymerization yields a polymer insoluble in NMP, which copolymer exhibits a swelling degree of 4 or below, in an electrolyte obtained by dissolving LiPF 6  in the concentration of 1 mol/liter into a solvent of 1:2 (volume ratio at 20° C.) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) . This binder for electrode of lithium ion secondary battery enables obtaining an electrode having a flexible electrode layer excelling in binding properties with industrial advantage.

TECHNICAL FIELD

The present invention relates to a binder for electrode of lithium ionsecondary battery; a slurry composition for electrode of lithium ionsecondary battery comprising the binder; an electrode for lithium ionsecondary battery; a production method for the electrode; and a lithiumion secondary battery having the electrode.

BACKGROUND ART

Secondary batteries such as a lithium ion secondary battery are widelyused as power sources of portable terminals, such as laptop computers,cellular phones and PDAs, which have remarkably spread in recent years.Recently, desires for extension of the operation time of portableterminals, and for shortening of the charging time thereof and the likehave been increasing. Accordingly, an improvement in the performance ofbatteries, in particular, getting high capacity thereof and animprovement in the charging and discharging rate (rate characteristic)thereof have been intensely desired.

A lithium ion secondary battery has a structure wherein a positiveelectrode and a negative electrode are arranged in such a manner that aseparator is interposed therebetween and the resultant is put, togetherwith an electrolyte, into a container. The electrodes (positiveelectrode and negative electrode) are those that an active material foran electrode (hereinafter referred to merely as an “active material”according to the circumstances) and an optional electroconductivitysupplying agent or the like are bonded to a current collector made ofaluminum, copper or the like through a binder for electrode of lithiumion secondary battery (hereinafter referred to merely as a “binder”according to the circumstances) . Usually, the electrodes are formed bydissolving or dispersing a binder into a liquid medium such as water orN-methylpyrrolidone (NMP) , mixing an active material and so ontherewith, applying the resultant slurry composition for electrode oflithium ion secondary battery (hereinafter referred to merely as a“slurry” according to the circumstances) onto a current collector, andthen removing the liquid medium by drying or the like to bond theresultant as an electrode layer thereto.

The battery capacity thereof is intensely affected by the amount of theactive material. The rate characteristic thereof is affected by theeasiness of electron transfer. In order to improve the ratecharacteristic, it is effective to increase the amount of theelectroconductivity supplying agent, such as carbon. In order toincrease the amounts of the active material and the electroconductivitysupplying agent inside a restricted space of the battery, it isnecessary to decrease the amount of the binder. However, if the amountof the binder is made small, there is caused a problem that the bondingforce of the electrode layer is reduced.

As a binder excellent in bonding power, there is known a copolymer madefrom an acrylic acid or methacrylic acid ester, acrylonitrile, and avinyl monomer having an acid component (see JP-A-8-287915). Thiscopolymer is insoluble in water and NMP; therefore, when a slurry isproduced therefrom, the copolymer is used in the state that theviscosity thereof is adjusted to a viscosity suitable for theapplication thereof onto a current collector, using a thickenertogether.

However, the concentration of solid contents in the slurry is usually ashigh as 70% or more, thereby resulting in a problem that mixing of theslurry becomes insufficient or the components thereof aggregate so thatthe active material or the electroconductivity supplying agent areunevenly dispersed in the slurry. If the uneven slurry is used to forman electrode, the following problems arise: the ion conductivity thereofdeteriorates so that the battery capacity lowers; and the bonding forceof the active material is reduced so that the active material falles offfrom the current collector.

As a method for obtaining a slurry wherein an active material and anelectroconductivity supplying agent are highly dispersed, suggested amethod of mixing a paste A obtained by kneading an active material and abinder, with a paste B, obtained by kneading an electroconductivitysupplying agent and a thickener, so as to yield a slurry (seeJP-A-2003-45432). However, according to this method, the processtherefor is complicated. Moreover, producing facilities become necessaryfor each of the pastes A and B and the slurry, and other largerestrictions about facilities are also imposed.

There is also known a method for using, as a binder, a polymer solublein NMP, such as polyvinylidene fluoride or polyacrylonitrile. However,the electrode produced by use of the binder is insufficient inflexibility; accordingly, its electrode layer may be cracked or fallenoff when the electrode is folded or wounded and then put into a batterycontainer.

DISCLOSURE OF THE INVENTION

Under such a situation, an object of the present invention is toprovide: a binder for electrode of lithium ion secondary battery whichis capable of obtaining, with industrial advantage, an electrode havinga flexible electrode layer good in bonding force; a slurry for lithiumion secondary battery electrode, comprising the binder; an electrode forlithium ion secondary battery; a production method for the electrode;and a lithium ion secondary battery having the electrode.

In order to solve the above-mentioned problems, the inventors have madeeager investigations so as to find out that a slurry wherein an activematerial and an electroconductivity supplying agent are highly dispersedand excelling in application, can be obtained by using, as a binder, acopolymer which comprises units of a monomer whose homopolymerizationyields a polymer soluble in NMP and units of a monomer whosehomopolymerization yields a polymer insoluble in NMP at a specific ratioand which exhibits a low swelling degree in a specific electrolyte.Furthermore, the inventors have found out that when the slurry is used,an electrode having a flexible electrode layer excellent in bondingforce can be obtained. On the basis of these findings, the presentinvention has been made.

Thus, according to the present invention, a binder for electrode oflithium ion secondary battery which comprises a copolymer comprising:

15 to 80 weight % of units from an ethylenically unsaturated monomer (A)whose homopolymerization yields a polymer soluble in N-methylpyrrolidone(NMP); and 20 to 85 weight % of units from an ethylenically unsaturatedmonomer (B) whose homopolymerization yields a polymer insoluble in NMP;which copolymer exhibits a swelling degree of 4 or below, in anelectrolyte obtained by dissolving LiPF₆ in the concentration of 1mole/liter into a solvent of 1:2 (volume ratio at 20° C.) mixture ofethylene carbonate (EC) and diethyl carbonate (DEC) is provided.

According to the invention, provided is also a binder for electrode oflithium ion secondary battery which comprises a copolymer obtained bymultistage-polymerizing: a component comprising at least oneethylenically unsaturated monomer whose homopolymerization yields apolymer soluble in NMP (this component being referred to as thecomponent (a)); and a component comprising at least one ethylenicallyunsaturated monomer whose homopolymerization yields a polymer insolublein NMP (this component being referred to as the component (b)); whichcopolymer exhibits a swelling degree of 4 or below, in an electrolyteobtained by dissolving LiPF₆ in the concentration of 1 mole/liter into asolvent of 1:2 (volume ratio at 20° C.) mixture of EC and DEC.

It is preferred that the multistage polymerization comprises a firstpolymerization step of polymerizing the component (a) and a subsequentsecond polymerization step of adding the component (b) thereto andpolymerizing these components. It is more preferred that the firstpolymerization step is a step of polymerizing 15 to 80 parts by weightof the component (a) until the polymerization conversion ratio thereofreaches 60 to 97 weight %, and the second polymerization step is a stepof adding 20 to 85 parts by weight of the component (b) thereto (whereinthe amount of all the monomers is 100 parts by weight) and polymerizingthe components until the polymerization conversion ratio reaches 90weight % or more of all the monomers.

It is also preferred that the multistage polymerization comprises athree-stage polymerization process. It is more preferred that themultistage polymerization comprises a first polymerization step ofadding a part of the component (a) and then polymerizing it, asubsequent second polymerization step of adding thereto the component(b) and polymerizing the components, and a subsequent thirdpolymerization step of adding thereto the remaining component (a) andpolymerizing the components. Furthermore, it is particularly preferredthat the first polymerization step is a step of polymerizing 5 to 50parts by weight of the component (a) until the polymerization conversionratio thereof reaches 60 to 97 weight %, the second polymerization stepis a step of adding 20 to 85 parts by weight of the component (b)thereto and polymerizing the components until the polymerizationconversion ratio reaches 60 to 97 weight % of all the monomers added upto this step, and the third polymerization step is a step of adding 5 to50 parts by weight of the component (a) thereto (wherein the amount ofall the monomers is 100 parts by weight) and polymerizing the componentsuntil the polymerization conversion ratio reaches 90 weight % or more ofall the monomers.

According to the invention, a slurry composition for electrode oflithium ion secondary battery which comprises the above-mentioned binderfor lithium ion secondary battery electrode, an active material for anelectrode, and an organic liquid medium is provided.

According to the invention, a production method for a lithium ionsecondary battery electrode, wherein the above-mentioned slurrycomposition for electrode of lithium ion secondary battery is appliedonto a current collector and then dried is provided.

According to the invention, a lithium ion secondary battery electrodewherein an electrode layer comprising the above-mentioned binder forelectrode of lithium ion secondary battery and an active material for anelectrode is bonded to a current collector; and a lithium ion secondarybattery which comprises this electrode are provided.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described along separated items in detailhereinafter.

(1) Binder For Electrode Of Lithium Ion Secondary Battery.

The binder of the present invention for lithium ion secondary battery isa binder comprises a copolymer comprising:

15 to 80 weight % of units from an ethylenically unsaturated monomer (A)whose homopolymerization yields a polymer soluble in N-methylpyrrolidone(NMP), and 20 to 85 weight % from units from an ethylenicallyunsaturated monomer (B) whose homopolymerization yields a polymerinsoluble in NMP, which copolymer exhibits a swelling degree of 4 orbelow, in an electrolyte obtained by dissolving LiPF₆ in theconcentration of 1 mole/liter in to a solvent of 1:2 (volume ratio at20° C.) mixture of EC and DEC.

Examples of the ethylenically unsaturated monomer (A), whosehomopolymerization yields a polymer soluble in NMP, includeα,β-ethylenically unsaturated nitrile compounds such as acrylonitrile,and methacrylonitrile;

aromatic vinyl compounds such as styrene, α-methylstyrene,β-methylstyrene, p-t-butylstyrene, and chlorostyrene; and

ethylenically unsaturated carboxylic acid esters wherein the alkyl groupbonded to the non-carbonyl oxygen atom has 6 or below carbon atoms, suchas methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate,n-butyl acrylate, isobutyl acrylate, methyl methacrylate, ethylmethacrylate, propyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, methyl crotonate, ethyl crotonate,propyl crotonate, butyl crotonate, and isobutyl crotonate. Of these, α,β-ethylenically unsaturated nitrile compounds and methyl methacrylateare preferred. These monomers may be used alone or in combination of twoor more thereof.

Examples of the ethylenically unsaturated monomer (B), whosehomopolymerization yields a polymer insoluble in NMP, includeethylenically unsaturated carboxylic acid esters wherein the alkyl groupbonded to the non-carbonyl oxygen atom has 7 or more carbon atoms, suchas 2-ethylhexyl acrylate, isooctyl acrylate, isodecyl acrylate, laurylacrylate, stearyl acrylate, tridecyl acrylate, 2-ethylhexylmethacrylate, isooctyl methacrylate, isodecyl methacrylate, laurylmethacrylate, tridecyl methacrylate, stearyl methacrylate, and2-ethylhexyl crotonate;

ethylenically unsaturated monocarboxylic acids such as acrylic acid,methacrylic acid, crotonic acid, and isocrotonic acid; ethylenicallyunsaturated dicarboxylic acids such as maleic acid, fumaric acid,citraconic acid, mesaconic acid, glutaconic acid, and itaconic acid;

conjugated dienes such as 1,3-butadiene, 2-methyl-1,3-butadiene(isoprene), 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene; and

1-olefins such as ethylene, propylene, and 1-butene. Of these,ethylenically unsaturated carboxylic acid esters wherein the alkyl groupbonded to the non-carbonyl oxygen atom has 7 or more carbon atoms andconjugated dienes are preferred to yield a binder excellent inflexibility. Particularly preferred are ethylenically unsaturatedcarboxylic acid esters wherein the alkyl group bonded to thenon-carbonyl oxygen atom has 7 or more carbon atoms.

The amount of the units of the ethylenically unsaturated monomer (A) isfrom 15 to 80 weight %, preferably from 20 to 75 weight %, morepreferably from 25 to 70 weight % of all the monomer units.

The amount of the units of the ethylenically unsaturated monomer (B) isfrom 20 to 85 weight %, preferably from 25 to 80 weight %, morepreferably from 30 to 75 weight % of all the monomer units.

In the case that the material of the binder comprises a copolymer madefrom the ethylenically unsaturated monomer (A) and the ethylenicallyunsaturated monomer (B), a good applicability when the binder is madeinto a slurry can be made compatible with the flexibility and thebonding force of the electrode produced by use of the slurry.

The binder of the present invention exhibits a swelling degree of 4 orbelow, preferably 3.5 or below, more preferably 3 or below in anelectrolyte obtained by dissolving LiPF₆ in the concentration of 1mole/liter into a solvent of 1:2 (volume ratio at 20° C.) mixture of ECand DEC. If the swelling degree is too large, the cycle characteristicand the rate characteristic are reduced. This would be based on thefollowing reason: when the binder swells, the bonding force lowersgradually so that the active material falls off from the currentcollector or the swelling binder covers the current collector to hinderthe transfer of electrons.

The swelling degree is measured by the following method: a cast filmmade of the binder is first formed in a usual way; the weight thereof isthen measured; thereafter, the film is immersed into the above-mentionedelectrolyte of 60° C. temperature; the immersed film is raised up after72 hours; the electrolyte adhering onto the surfaces of the film iswiped with a paper towel; immediately the weight of the film ismeasured; and the swelling degree is calculated as a value of (theweight after the immersion)/(the weight before the immersion).

The production method for the binder is not particularly limited.Preferably, the binder can be obtained by multistage-polymerizing theethylenically unsaturated monomer (A) and the ethylenically unsaturatedmonomer (B).

The wording “multistage-polymerizing” or “multistage polymerization” inthe invention means that a part of a monomer is first polymerized,subsequently a monomer different therefrom in kind and/or blend ratio isadded thereto, and the components are polymerized. The wording“‘subsequently’ the components are polymerized” means that in the statethat the monomer in the previous polymerization step remains, that is,at the time when the polymerization conversion ratio does not become100%, subsequent polymerization is performed.

The binder of the present invention is also a binder which comprises acopolymer obtained by multistage-polymerizing a component comprising atleast one ethylenically unsaturated monomer whose homopolymerizationyields a polymer soluble in NMP (this component being referred to as thecomponent (a)), and a component comprising at least one ethylenicallyunsaturated monomer whose homopolymerization yields a polymer insolublein NMP (this component being referred to as the component (b)), whichcopolymer exhibits a swelling degree of 4 or below in an electrolyteobtained by dissolving LiPF₆ in the concentration of 1 mole/liter into asolvent of 1:2 (volume ratio at 20° C.) mixture of EC and DEC.

The monomer used as each of the component (a) and the component (b) maybe one monomer or a mixture of two or more monomers. In the case ofusing the mixture of two or more monomers, the component (a) may containthe ethylenically unsaturated monomer (B) as long as the polymer yieldedby polymerizing the component (a) is soluble in NMP. The component (b)may contain the ethylenically unsaturated monomer (A) as long as thepolymer yielded by polymerizing the component (b) is insoluble in NMP.In other words, when the copolymer yielded by copolymerizing theabove-mentioned monomer mixture is soluble in NMP, the monomer mixturecan be used as the component (a). When the copolymer is insoluble inNMP, the monomer mixture can be used as the component (b).

The multistage polymerization is preferably performed through two orthree steps, and is more preferably performed through three steps. Ifthe polymerization is performed through four or more steps, the processbecomes complicated so that the productivity may fall.

When the polymerization is performed through two steps, thepolymerization is preferably multistage polymerization having a firstpolymerization step of polymerizing the component (a) and a subsequentsecond step of adding the component (b) and polymerizing the components.The amount of the component (a) in the first polymerization step ispreferably from 15 to 80 parts by weight, more preferably from 20 to 75parts by weight, even more preferably from 25 to 70 parts by weight. Theamount of the component (b) in the second polymerization step ispreferably from 20 to 85 parts by weight, more preferably from 25 to 80parts by weight, even more preferably from 30 to 75 parts by weight(wherein the amount of all the monomers is 100 parts by weight).

The polymerization conversion ratio in the first polymerization step ispreferably from 60 to 97 weight %, more preferably from 65 to 97 weight%, even more preferably from 70 to 95 weight %. The polymerizationconversion ratio in the second polymerization step is preferably 90weight % or more, more preferably 95 weight % or more of all themonomers.

When the polymerization is performed through three steps, thepolymerization is preferably multistage polymerization having a firstpolymerization step of adding the component (a) and polymerizing thecomponent, a subsequent second polymerization step of adding thereto thecomponent (b) and polymerizing the components, and a subsequent thirdpolymerization step of adding thereto the component (a) and polymerizingthe components. In this case, the component (a) used in the firstpolymerization step may be equal to or different from the component (a)used in the third polymerization step.

The amount of the component (a) in the first polymerization step ispreferably from 5 to 50 parts by weight, more preferably from 5 to 45parts by weight, even more preferably from 10 to 40 parts by weight; theamount of the component (b) in the second polymerization step ispreferably from 20 to 85 parts by weight, more preferably from 25 to 80parts by weight, even more preferably from 30 to 75 parts by weight; andthe amount of the component (a) in the third polymerization step ispreferably from 5 to 50 parts by weight, more preferably from 5 to 45parts by weight, even more preferably from 10 to 40 parts by weight(wherein the amount of all the monomers is 100 parts by weight).

In each of the first and second polymerization steps, the polymerizationconversion ratio is preferably from 60 to 97 weight %, more preferablyfrom 65 to 97 weight %, even more preferably from 70 to 95 weight % ofall the monomers added up to the step. In the third polymerization step,the polymerization conversion ratio is preferably 90 weight % or more,more preferably 95 weight % or more of all the monomers.

The binder yielded by the multistage polymerization gets satisfactory inbonding power, and the slurry yielded by use of this binder becomes abinder good in applicability wherein an active material and anelectroconductivity supplying agent are highly dispersed.

The polymerization method for the binder of the invention is notparticularly limited, and a known polymerization method may be adopted,an example of which is emulsion polymerization, suspensionpolymerization, dispersion polymerization, or solution polymerization.Of these, emulsion polymerization is preferred.

(2) Slurry Composition For Electrode Of Lithium Ion Secondary Battery.

The slurry composition of the invention for electrode of lithium ionsecondary battery comprises the above-mentioned binder, an activematerial for an electrode, and an organic liquid medium. The organicliquid medium is not particularly limited if the medium is a liquidmedium wherein the binder can be dissolved or dispersed into the form offine particles. Specific examples thereof include amides such asN-methylpyrrolidone, N,N-dimethylacetoamide, and dimethylformamide. Ofthese, N-methylpyrrolidone is particularly preferred because of beingexcelling in application onto a current collector and dispersion of thebinder.

The method for dissolving or dispersing the binder of the invention intothe organic liquid medium is not particularly limited. When the binderof the invention is produced into a latex form by emulsionpolymerization, the method is, for example, a method of substituting aspecific organic liquid material for water in the latex. An example ofthe method for the substitution is a method of adding an organicdispersion medium to the latex and then removing the water in thedispersion medium by distillation, a dispersion medium phase convertingmethod or the like.

The amount of the organic liquid medium is adjusted in such a mannerthat the slurry composition gives a viscosity suitable for theapplication thereof in accordance with the kinds of the binder, and anactive material and an electroconductivity supplying agent which will bedescribed later. The concentration of solid contents in the total of thebinder, the active material and the electroconductivity supplying agentis preferably from 50 to 90 weight %, more preferably from 70 to 90weight %.

The active material used in the slurry of the invention is appropriatelyselected in accordance with the kind of the electrode. The slurry of theinvention can be used for any one of a positive electrode and a negativeelectrode, and is preferably used for a positive electrode. The activematerial may be any material that can be used in an ordinary lithium ionsecondary battery. Examples of the active material for a positiveelectrode include lithium-containing composite metal oxides such asLiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, LiFePO₄, and LiFeVO₄; transition metalsulfides such as TiS₂, TiS₃, and amorphous MOS₃; and transion metaloxides such as Cu₂V₂O₃, amorphous V₂O—P₂O₅, MoO₃, V₂O₅, and V₆O₁₃.Furthermore, an electroconductive polymer such as polyacetylene orpoly-p-phenylene may be used.

Examples of the active material for a negative electrode includecarbonous materials such as amorphous carbon, graphite, naturalgraphite, mesocarbon microbeads (MCMB), and pitch-based carbon fiber;and electroconductive polymers such as polyacene. The shape and the sizeof the active material are not particularly limited. The active materialhaving a surface onto which an electroconductivity supplying agent isbonded by a mechanically reforming method may be used.

The slurry of the invention may contain, besides the binder of theinvention, other binders different therefrom. The use of the otherbinder together makes it possible to adjust the viscosity or thefluidity of the slurry and the bonding force or flexibility of theelectrode obtained by use of the slurry within broader ranges. The ratioof the amount of the binder of the invention to that of the other binderis not particularly limited, and the ratio by weight is preferably from5:1 to 1:5, more preferably from 3:1 to 1:3.

Examples of the other binder include cellulose polymers such ascarboxymethyl cellulose, methyl cellulose and hydroxypropylcellulose,and ammonium salts and alkali metal salts thereof; homopolymers eachmade from an α,β-ethylenically unsaturated nitrile compound, such aspolyacrylonitrile or polymethacrylonitrile; copolymers each made from anα,β-ethylenically unsaturated nitrile compound and an 1-olefin,ethylenically unsaturated carboxylic acid or ethylenically unsaturatedcarboxylic acid ester; acrylic rubbers such as 2-ethylhexylacrylate/methacrylic acid/acrylonitrile/ethylene glycol dimethacrylatecopolymer, and butyl acrylate/acrylic acid/trimethylolpropanetrimethacrylate copolymer; acrylonitrile/butadiene rubber, andhydrogenated product thereof; vinyl alcohol based polymers such asethylene/vinyl alcohol copolymer, and vinyl alcohol/vinyl acetatecopolymer; and fluorine-contained polymers such as polyvinylidenefluoride, polytetrafluoroethylene, and polypentafluoropropylene.

If necessary, an electroconductivity supplying agent is added to theslurry of the invention. The electroconductivity supplying agent thatcan be used may be a carbonous material such as graphite, activatedcarbon, acetylene black, ketchen black, furnace black, graphite, carbonfiber or a fullerene, an electroconductive polymer or a metal powder. Ofthese, acetylene black or furnace black is preferred. The amount of theused electroconductivity supplying agent is usually from 0.5 to 20 partsby weight, preferably from 1 to 10 parts by weight for 100 parts byweight of the active material.

If necessary, a viscosity adjustor, a fluidization agent or othercomponents may be added to the slurry of the invention.

The slurry of the invention is produced by mixing the respectivecomponents. The mixing method thereof and the mixing order thereof arenot particularly limited. When the binder of the invention is used, aslurry wherein the active material and the electroconductivity supplyingagent are highly dispersed can be obtained regardless of the mixingmethod and the mixing order. For the mixing, a mixer may be used,examples of the mixer including a ball mill, a sand mill, a pigmentdisperser, a crusher, an ultrasonic disperser, a homogenizer, and aplanetary mixer.

(3) Electrode Of Lithium Ion Secondary Battery.

The electrode of lithium ion secondary battery of the invention is anelectrode wherein an electrode layer comprising the above-mentionedbinder and an active material for an electrode is bonded to a currentcollector.

The current collector is not particularly limited if the collector ismade of an electroconductive material. Examples thereof include metalssuch as iron, copper, aluminum, nickel, and stainless steel. Inparticular, when aluminum is used for positive electrode and copper isused for negative electrode, the advantageous effects of the slurry ofthe invention are most satisfactorily exhibited. The shape of thecurrent collector is not particularly limited, and the current collectoris preferably in the form of a sheet having a thickness from about 0.001to 0.5 mm.

(4) Production Method For An Electrode For Lithium Ion Secondary Battery

The electrode of the invention can be produced by applying the slurry ofthe invention onto a current collector, drying the slurry and thusbonding the resultant electrode layer, which comprises a binder, anactive material, and optional components such as an electroconductivitysupplying agent, onto the collector.

The method for applying the slurry onto the current collector is notparticularly limited. Examples thereof include doctor blade, dipping,reverse roll, direct roll, gravure, and brush-painting methods. Theamount of the applied slurry is not particularly limited, and isgenerally such an amount that the thickness of the electrode layer,which is formed after the organic liquid medium is dried and removed andcomprises the active material, the binder and so on, can be set usuallyinto the range of 0.005 to 5 mm, preferably into the range of 0.01 to 2mm. The method for the drying is not particularly limited, and examplesthereof include drying with warm wind, hot wind, low humid wind, vacuumdrying, and a drying method based on the radiation of (far) infraredrays, electron beams, or the like. The speed for the drying is adjustedin such a manner that the liquid medium can be removed as rapidly aspossible within such a speed range that the electrode layer is notcracked by stress concentration and the electrode layer is not fallenoff from the current collector. The dried current collector may bepressed, thereby heightening the density of the active material in theelectrode. Examples of the method for the pressing include mold-pressingand roll-pressing.

(5) Lithium Ion Secondary Battery

The lithium ion secondary battery of the invention is a batterycomprising the above-mentioned electrode for lithium ion secondarybattery.

The lithium ion secondary battery can be produced by an ordinary methodusing the above-mentioned electrode, an electrolyte, and members such asa separator. In a specific example of the production process, a negativeelectrode and a positive electrode are overlapped with each other in thestate that a separator is interposed therebetween, and this is woundedor folded in accordance with the battery shape and put into a batterycontainer. An electrolyte is poured into the battery container and thenthe container is sealed up. If necessary, an expanding metal, anovercurrent protecting element such as electric fuse or PTC element, alead plate, and so on are put into the container, thereby making itpossible to prevent a rise in the pressure inside the battery andovercharge/overdischarge. The shape of the battery may the shape of acoin, a button, a sheet, a cylinder, rectangular, flat or any othershape.

The electrolyte may be in a gel or liquid form if the electrolyte is anelectrolyte that can be used in an ordinary lithium ion secondarybattery. It is advisable to select an electrolyte which causes batteryfunction to be exhibited in accordance with the kinds of the negativeactive material and the positive active material.

As the electrolyte, any known lithium salt can be used. Examples thereofinclude LiClO₄, LiBF₄, LiPF₆, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiB₁₀Cl₁₀,LiAlCl₄, LiCl, LiBr, LiB(C₂H₅)₄, LiCF₃SO₃, LiCH₃SO₃, LiC₄F₉S₃,Li(CF₃SO₂)₂N, and lower aliphatic acid carboxylic acid lithium salts.

The medium for dissolving these electrolytes (electrolyte solvent) isnot particularly limited. Specific examples thereof include carbonatessuch as propylene carbonate, ethylene carbonate, butylene carbonate,dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate;lactones such as γ-lactone; ethers such as trimethoxymethane,1,2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran, and2-methyltetrahydrofuran; and sulfoxides such as dimethylsulfoxide. Ofthese, carbonates are preferred since they are excellent in chemical,electrochemical and thermal stabilities.

These medium may be used alone or in the form of a mixed solvent of twoor more thereof. The binder of the invention has a low swelling degreein carbonates; therefore, it does not happen that on the basis of theswelling of the binder, the bonding force thereof falls, whereby theactive material falls off from the current collector or that the bindercovers the current collector to hinder the transfer of electrons.

EXAMPLES

The present invention will be described by way of the following workingexamples. However, the invention is not limited thereto. In the workingexamples, the expressions “part(s)” and “%” are based on “weight”,unless otherwise specified.

Tests and evaluations in the working examples and comparative exampleswere made by the following methods.

(1) Polymerization Conversion Ratio

The polymerization conversion ratio at the time of polymerizationreaction was obtained from the weight of solid contents obtained bydrying a dispersion of the polymer in water by calculation. Thepolymerization conversion ratio represents the polymerization conversionratio to the total amount of the monomer(s) added up to the stage.

(2) Swelling Degree In An Electrolyte

The swelling degree of a polymer used as a binder in an electrode wasobtained by the following method:

A liquid which is 0.2 g of the polymer was dissolved or dispersed in 10mL of N-methylpyrrolidone was caused to flow and extend onto a sheetmade of polytetrafluoroethylene, and then dried to yield a cast film. Apiece of 4 cm² area was cut out from this cast film, and the weightthereof was measured. Thereafter, the piece was immersed into anelectrolyte of 60° C. temperature. The immersed film was raised up after72 hours, and then the electrolyte adhering onto the film surfaces waswiped with a paper towel. Immediately, the weight of the film wasmeasured. The value of (the weight after the immersion)/(the weightbefore the immersion) was defined as the swelling degree. As theelectrolyte, there was used an electrolyte obtained by dissolving LiPF₆in the concentration of 1 mole/liter into a solvent of 1:2 (volume ratioat 20° C.) mixture of ethylene carbonate (EC) and diethyl carbonate(DEC).

(3) Composition Ratio In A Polymer

The ratio between units from an ethylenically unsaturated monomer (A)capable of giving a homopolymer soluble in NMP (an NMP solublecomponent) and units from an ethylenically unsaturated monomer (B)capable of giving a homopolymer insoluble in NMP (an NMP insolublecomponent) was obtained by ¹H— and ¹³C-NMR measurements.

(4) Peel Strength

<Production Of A Positive Electrode>

A slurry for a positive electrode was applied evenly onto one surface ofan aluminum foil (thickness: 20 μm) by a doctor blade method. Theresultant was dried at 120° C. in a drying machine for 45 minutes.Furthermore, the resultant was dried at 120° C. under a reduced pressureof 0.6 kPa in a vacuum drying machine for 2 hours, and then pressed witha biaxial roll press so as to set the electrode density into 3.3 g/cm³.In this way, a positive electrode was yielded.

<Production Of A Negative Electrode>

A slurry for a negative electrode was applied evenly onto one surface ofan aluminum foil (thickness: 18 μm) by a doctor blade method. Theresultant was dried under the same conditions as about the positiveelectrode. The resultant was then pressed with a biaxial roll press soas to set the electrode density into 1.4 g/cm³. In this way, a negativeelectrode was yielded.

<Measurement Of Peel Strength>

The electrode (the positive electrode or negative electrode) obtained bythe above-mentioned method was cut into a rectangular piece 2.5 cm wideand 10 cm long. The piece was fixed in the state that its electrodelayer surface was directed upwards. A cellophane tape was stuck onto theelectrode layer surface. The stress (N/cm) generated when the electrodewas fixed and the tape was peeled off at a rate of 50 mm/minute in thedirection of 180° angle was measured 10 times. The average value thereofwas calculated, and the resultant was defined as the peel strength. Asthis value is larger, the bonding power is higher and it is moredifficult to fall off the active material from the current collector.

(5) Bending Test

The electrode obtained by the method in the item (4) was cut intorectangular pieces 3 cm wide and 9 cm long to be used as test pieces.The test piece was put onto a desk, and a stainless steel round bar of 1mm diameter was set onto the current collector side surface thereof inthe state that the bar was positioned at the piece center in thelongitudinal direction (at a place 4.5 cm apart from ends thereof) andwas laid along the short direction. The test piece was bent at 180°around this stainless bar. The state of the coating film (electrodelayer) on the outside of the bent portion was observed. 10 test pieceswere tested. A case wherein a crack or falling was not generated at allin the ten pieces was judged as “good”. A case wherein one or more spotswere cracked or fallen in at least one of the pieces was judged as“poor”. The matter that the electrode layer is not cracked or fallendemonstrates that the electrode is excellent in flexibility.

(6) Battery Capacity, And Charge And Discharge Cycle Characteristic

<Production Of A Lithium Ion Secondary Battery (For Evaluating APositive Electrode)>

For evaluating a positive electrode, metal lithium was used as anegative electrode.

The positive electrode produced in the method described in the item (4)was cut out into a circle of 15 mm diameter. A separator made of acircular polypropylene porous film of 18 mm diameter and 25 μmthickness, lithium metal as the negative electrode, and an expandingmetal were successively laminated onto the electrode layer side face ofthis positive electrode. This was put into a coin-shaped outer-packagingcontainer (diameter: 20 mm, height: 1.8 mm, and stainless steelthickness: 0.25mm) , made of stainless steel, to which a polypropylenepacking was fitted. An electrolyte was injected into this containerwithout leaving any air. A stainless steel cap of 0.2 mm thickness wasput and fixed, through the polypropylene packing, onto theouter-packaging container. The battery can was sealed up to produce acoin-shaped battery (for evaluating the positive electrode) of 20 mmdiameter and about 2 mm thickness. The used electrolyte was the same asused to measure the swelling degree.

<Production Of A Lithium Ion Secondary Battery (For Evaluating ANegative Electrode)>

For evaluating a negative electrode, metal lithium was used as apositive electrode.

The negative electrode produced in the method described in the item (4)was cut out into a circle of 15 mm diameter. A separator, lithium metalas the positive electrode, and an expanding metal were successivelylaminated onto the electrode layer side face of this negative electrode.This was put into a coin-shaped outer-packaging container, andsubsequent steps were carried out in the same way as in the productionof the positive-evaluating battery, so as to produce a coin-shapedbattery (for evaluating the negative electrode). As the separator, thecoin-shaped outer-packaging container, and the electrolyte, the same forevaluating the positive electrode were used.

<Measurements of Battery Capacity And Charge And Discharge CycleCharacteristic>

The coin-shaped batteries produced by the above-mentioned methods wereused to repeat charge and discharge by a 0.1 C constant electric currentmethod at 23° C. and at voltages from 3 to 4.2 V for positive electrodeevaluation and at voltages from 0 to 1.2 V for negative electrodeevaluation, respectively. The discharge capacities at a 3^(rd) cycle andat a 50^(th) cycle were measured. The discharge capacity thereof in the3^(rd) cycle was defined as the battery capacity. The unit thereof ismAh/g (of the active material).

The ratio of the discharge capacity in the 50^(th) cycle to that in the3^(rd) cycle was calculated by percentage. As this value is larger, areduction in the capacity is smaller and thus the charge and dischargecycle characteristic is better.

(8) Charge And Discharge Rate Characteristic

The discharge capacity in the 3^(rd) cycle in each constant currentamount was measured in the same way as in the measurement of batterycapacity except that the measurement condition was changed to 1 C ofconstant electric current amount. The rate of the discharge capacity in1 C to that in 0.1 C in the 3^(rd) cycle was calculated by percentage.As this value is larger, the faster charge and discharge can be done andthus the charge and discharge rate characteristic is better.

EXAMPLE 1

400 parts of ion-exchange water, 26 parts of acrylonitrile, 5 parts ofsodium dodecylbenzenesulfonate and 3 parts of potassium persulfate werecharged into an autoclave with a stirrer, and then the solution wassufficiently stirred. Thereafter, the solution was heated to 60° C. toinitiate polymerization at a first stage. When the polymerizationconversion ratio reached 85%, thereto were added 48 parts of2-ethylhexyl acrylate as a monomer for a second stage to continue thereaction. When the polymerization conversion ratio at the second stagereached 90%, thereto were added 26 parts of acrylonitrile as a monomerfor a third stage. When the polymerization conversion ratio reached 99%,the solution was cooled to stop the polymerization. Lithium hydroxidewas added to the latex yielded in the above-mentioned three-stagepolymerization to adjust the pH thereof to 7. Next, thereto was addedN-methylpyrrolidone (NMP) in an amount 3 times larger than the totalweight of the latex, and then water therein was volatilized with anevaporator to yield a dispersion of a polymer A-1 in NMP, having a solidcontent concentration of 8%.

Into a planetary mixer were charged 100 parts of lithium cobaltate as anactive material and 3 parts of acetylene black (Denka Black,manufactured by Denki Kagaku Kogyo Co., Ltd.) as an electroconductivitysupplying agent. Thereto was added NMP to set the solid contentconcentration to 90%, and the resultant was stirred for 20 minutes tomix the components. Thereafter, to the solution was added one part ofthe solution of the polymer A-1 in NMP. The amount of the solution wasan amount based on solid contents. The resultant was kneaded at a solidcontent concentration of 82% for 90minutes. Thereafter, to the resultantwas added NMP to adjust the slurry viscosity. This slurry was used toform a positive electrode. The composition and the swelling degree ofthe polymer A-1, and characteristics of the resultant electrode andbattery were measured. The results are shown in Table 1.

EXAMPLE 2

Polymerization at a first stage was initiated in the same way as inExample 1 except that the amount of acrylonitrile for the first stagewas changed to 40 parts. When the polymerization conversion ratioreached 90%, thereto were added a mixture of 58 parts of 2-ethylhexylacrylate and 2 parts of methacrylic acid as monomers for a second stage.When the polymerization conversion ratio reached 98%, the slurry wascooled to stop the polymerization. In this way, a latex was yielded bythe two-stage polymerization. Thereafter, the same way as in Example 1was carried out to yield a dispersion of a polymer A-2 in NMP, having asolid content concentration of 8 weight %. This polymer A-2 was used toform a positive electrode. The composition and the swelling degree ofthe polymer A-2, and characteristics of the electrode and batteryobtained by use of this polymer were measured. The results are shown inTable 1.

EXAMPLE 3-5

Polymers A-3 to A-5 were each yielded in the same way as in Example 1except that each formulation shown in Table 1 was used. The compositionand the swelling degree of each of the resultant polymers, andcharacteristics of the electrode and battery produced in the same way asin Example 1 by use of the polymer were measured. The results are shownin Table 1.

EXAMPLE 6

A polymer A-6 was yielded in the same way as in Example 2 except that aformulation shown in Table 1 was used. The composition and the swellingdegree of the resultant polymer, and characteristics of the electrodeand battery produced in the same way as in Example 1 by use of thepolymer were measured. The results are shown in Table 1.

EXAMPLE 7

A dispersion of a polymer A-7 in NMP was yielded in the same way as inExample 1 except that a formulation shown in Table 1 was used.

Into a planetary mixer were charged 1.3 parts of the solution of thepolymer A-7 in NMP, the amount of the solution being based on solidcontents, and 96 parts of mesocarbon microbeads as an active material,and then thereto was added NMP so as to set the solid contentconcentration to 65%. The resultant slurry was stirred to mix thecomponents therein. This slurry was used to form a negative electrode.The composition and the swelling degree of the resultant polymer A-7,and characteristics of the electrode and battery yielded by use of thepolymer were measured. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

A polymer B-1 was yielded in the same way as in Example 1 except that apolymerization conversion ratio in each polymerization stage was changedas shown in Table 1. The resultant polymer was dissolved in theelectrolyte (the swelling degree thereof was infinitely large) . Thecomposition and the swelling degree of the resultant polymer, andcharacteristics of the electrode and battery produced in the same way asin Example 1 by use of the polymer were measured. The results are shownin Table 1.

COMPARATIVE EXAMPLE 2

400 parts of ion-exchange water, 40 parts of acrylonitrile, 58 parts of2-ethylhexyl acrylate, 2 parts of methacrylic acid, 5 parts of sodiumdodecylbenzenesulfonate and 3 parts of potassium persulfate were chargedinto an autoclave with a stirrer, and then the solution was sufficientlystirred. Thereafter, the solution was heated to 60° C. to initiatepolymerization. When the polymerization conversion ratio reached 96%,the slurry was cooled to stop the polymerization. In this way, a latexwas yielded by one-stage polymerization. Thereafter, a polymer B-2 wasyielded in the same way as in Example 1. The resultant polymer wasdissolved in the electrolyte (the swelling degree thereof was infinitelylarge) . The composition and the swelling degree of the resultantpolymer, and characteristics of the electrode and battery produced inthe same way as in Example 1 by use of the polymer were measured. Theresults are shown in Table 1.

COMPARATIVE EXAMPLE 3

A polymer B-3 was yielded in the same way as in Comparative Example 2except that a formulation shown in Table 1 was used. In the resultantpolymer, the composition ratio of the NMP soluble component (A)/theinsoluble component (B) was 12/88. This polymer B-3 was used to attemptto form a positive electrode in the same way as in Example 1. However,the fluidity of the slurry was bad. At the time of press, the coatingfilm was fallen off from the current collector. As a result, noelectrode was permitted to be produced.

COMPARATIVE EXAMPLE 4

400 parts of ion-exchange water, 0.3 part of partially-saponificatedpolyvinyl alcohol, 0.2 part of methylcellulose, 0.2 part ofazobisisobutyronitrile, and 100 parts of acrylonitrile were charged intoan autoclave with a stirrer, so as to continue reaction at 80° C. for 8hours, thereby yielding a homopolymer B-4 of acrylonitrile. Thecomposition and the swelling degree of the resultant polymer, andcharacteristics of the electrode and battery produced in the same way asin Example 1 by use of the polymer were measured. The results are shownin Table 1. TABLE 1 Examples 1 2 3 4 5 6 7 Binder A-1 A-2 A-3 A-4 A-5A-6 A-7 First-polymerization components (parts) 2-Ethylhexyl acrylateAcrylonitrile 26 40 20 16 25 Methacrylic acid Methyl methacrylate 20 40First-polymerization conversion ratio (%) 85 90 70 81 72 88 85Second-polymerization components (parts) 2-Ethylhexyl acrylate 48 58 5868 60 Dodecyl acrylate 60 Butadiene 55 Methacrylic acid 2 2Second-polymerization conversion ratio (%) 90 98 85 80 85 97 90Third-polymerization component (parts) Acrylonitrile 26 20 16 20 20Third-polymerization conversion ratio (%) 99 97 95 97 96 Compositionratio in the polymer (NMP soluble 52/48 40/60 40/60 32/68 40/60 40/6045/55 component/insoluble component) Swelling degree in the electrolyte1.7 1.8 2 1.8 2.2 2 1.6 Electrode characteristics Electrode kindPositive Positive Positive Positive Positive Positive Negative electrodeelectrode electrode electrode electrode electrode electrode Used binderamount (part(s)) 1 1 1 1 1 1 1.3 Peel strength (N/cm) 0.09 0.10 0.100.08 0.08 0.08 0.10 Bending test Good Good Good Good Good Good GoodBattery characteristics Battery capacity(mAh/g) 145 147 145 146 142 145345 Charge and discharge cycle characteristic(%) 69 66 63 65 65 68 75Charge and discharge rate characteristic(%) 49 42 43 46 45 48 65Comparative Examples 1 2 3 4 Binder B-1 B-2 B-3 B-4 First-polymerizationcomponents (parts) 2-Ethylhexyl acrylate 58 83 Acrylonitrile 26 40 12100 Methacrylic acid 2 5 Methyl methacrylate First-polymerizationconversion ratio (%) 55 96 98 97 Second-polymerization components(parts) 2-Ethylhexyl acrylate 48 Dodecyl acrylate Butadiene Methacrylicacid Second-polymerization conversion ratio (%) 50 Third-polymerizationcomponent (parts) Acrylonitrile 26 Third-polymerization conversion ratio(%) 98 Composition ratio in the polymer (NMP soluble 52/48 40/60 12/88100/0 component/insoluble component) Swelling degree in the electrolyteDissolved Dissolved 2.8 1.3 Electrode characteristics Electrode kindPositive Positive Unable to be Positive electrode electrode producedelectrode Used binder amount (part(s)) 1 1 — 1.5 Peel strength (N/cm)0.08 0.10 — 0.10 Bending test Good Good — Poor Battery characteristicsBattery capacity(mAh/g) 138 127 — 138 Charge and discharge cyclecharacteristic(%) 39 40 — 38 Charge and discharge rate characteristic(%)28 25 — 28

PRODUCTION EXAMPLE 1

A polymer C was yielded in the same way as in Comparative Example 2except that a formulation shown in Table 2 was used. The composition andthe swelling degree of the resultant polymer are shown in Table 2.

PRODUCTION EXAMPLE 2

A polymer D was yielded in the same way as in Comparative Example 4except that 97 parts of acrylonitrile and 3 parts of acrylic acid wereused instead of 100 parts of acrylonitrile. The composition and theswelling degree of the resultant polymer are shown in Table 2.

PRODUCTION EXAMPLE 3

A polymer E was yielded in the same way as in Production Example 2except that the amount of acrylonitrile was set to 90 parts and 10 partsof methyl acrylate were used instead of 3 parts of acrylic acid. Thecomposition and the swelling degree of the resultant polymer are shownin Table 2. TABLE 2 C D E F Polymer composition (weight %) 2-Ethylhexylacrylate 86 Acrylonitrile 9 97 90 Acrylic acid 4 3 Diethylene glycoldimethacrylate 1 Methyl acrylate 10 Vinylidene fluoride 100 Compositionratio in the 9/91 97/3 90/10 100/0 polymer (NMP solublecomponent/insoluble component) Swelling degree in the electrolyte 1.71.4 1.6 1.3

EXAMPLE 8

100 parts of lithium cobaltate as an active material and 3 parts ofacetylene black (manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) asan electroconductivity supplying agent were charged into a planetarymixer. Thereto was added NMP to set the solid content concentration to90%, and the resultant was stirred for 20 minutes to mix the components.Thereafter, to this solution was added 0.5 part of the dispersion of thepolymer A-1 in NMP yielded in Example 1 and 0.5 part of the dispersionof the polymer C having a composition and a swelling degree shown inTable 2. The amounts of the dispersions were amounts based on solidcontents. The resultant was kneaded at a solid content concentration of82% for 90 minutes. Furthermore, to the resultant was added NMP toadjust the slurry viscosity. This slurry was used to form a positiveelectrode. The ratio of the NMP soluble component to the NMP insolublecomponent in all of the used binders and characteristics of theresultant electrode and battery were measured. The results are shown inTable 3.

EXAMPLES 9-11, AND COMPARATIVE EXAMPLES 5 AND 6

Electrodes were each formed in the same way as in Example 8 except thatthe kind and the amount of the polymer used as a binder were renderedthose shown in Table 3. As the polymer F, a polyvinylidene fluoride,#1100 (manufactured by Kureha Chemical Industry Co., Ltd.) was used. Theratio of the NMP soluble component to the NMP insoluble component in allof the used binders and characteristics of the resultant electrode andbattery were measured. The results are shown in Table 3. TABLE 3Comparative Examples Examples 8 9 10 11 5 6 Binder kind A-1 A-1 A-2 A-4B-3 B-4 Used binder amount (part) 0.5 0.7 0.7 0.4 0.7 0.3 Binder usedtogether PolymerC(part) 0.5 0.5 0.7 PolymerD(part) 0.3 PolymerE(part)0.3 PolymerF(part) 0.3 0.3 Binder composition ratio 31/69 66/34 55/4539/61 38/62 36/62 (NMP soluble component/insoluble component) Electrodecharacteristics Peel strength(N/cm) 0.11 0.12 0.10 0.10 0.06 0.06Bending test Good Good Good Good Good Good Battery characteristicsBattery capacity(mAh/g) 143 144 141 146 120 131 Charge and dischargecycle characteristic(%) 67 67 69 70 32 45 Charge and discharge ratecharacteristic(%) 44 43 46 48 25 31

The electrodes produced by use of the binder of the invention exhibitedan excellent bonding force and a high flexibility whether the binder wasused alone or used together with a different binder. The lithium ionsecondary batteries each having one of these electrodes had a highbattery capacity and a good cycle characteristic and further wereexcellent in the rate characteristic (Examples 1 to 11). On the otherhand, in cases where a binder soluble in the electrolyte was used, theflexibility of the electrodes was high but the batteries producedtherefrom were poor in resistance against the electrolyte; therefore,the charge and discharge cycle characteristic and the charge anddischarge rate characteristic thereof lowered (Comparative Examples 1and 2).

When a binder wherein the amount of the component insoluble in NMP wastoo large was used, an even slurry was not easily yielded and thus noelectrode was permitted to be produced (Comparative Example 3). On theother hand, when a binder wherein the amount of the component soluble inNMP was too large was used, the resultant electrode was poor inflexibility so that its electrode layer might be cracked or fallen. Thebattery produced therefrom exhibited a low charge and discharge cyclecharacteristic and a low charge and discharge rate characteristic(Comparative Example 4). When a binder wherein the amount of thecomponent soluble in NMP was too large and a binder wherein the amountof the component insoluble in NMP was too large were used together, aneven slurry was not easily yielded and thus a poor bonding force wasexhibited. The performance of the batteries yielded therefrom was alsopoor (Comparative Examples 5 and 6).

Some of the embodiments of the present invention have been describedabove by way of the above-mentioned working examples. It is evident forthose skilled in the art that embodiments wherein the working examplesare modified can be carried out as long as the embodiments do not departfrom the subject matter and the concept of the present invention. Suchmodified embodiments are included in the scope of the invention. Theabove-mentioned comparative examples have been described in order todemonstrate that the working examples are concerned with excellentembodiments by the comparison between the working examples and thecomparative examples. Accordingly, the objects of the invention may beattained in accordance with the contents of the comparative examples.

INDUSTRIAL APPLICABILITY

The use of the binder of the invention, for lithium ion secondarybattery electrode, makes it possible to yield easily an electrode havinga flexible electrode layer good in bonding force. This electrode isexcellent in electrolyte resistance; accordingly, the lithium ionsecondary battery having this electrode has a high charge and dischargecapacity and a good cycle characteristic, and also exhibits an excellentrate characteristic.

1. A binder for electrode of lithium ion secondary battery, whichcomprises a copolymer comprising: 15 to 80 weight % of units from anethylenically unsaturated monomer (A) whose homopolymerization yields apolymer soluble in N-methylpyrrolidone (NMP); and 20 to 85 weight % ofunits from an ethylenically unsaturated monomer (B) whosehomopolymerization yields a polymer insoluble in NMP; which copolymerexhibits a swelling degree of 4 or below in an electrolyte obtained bydissolving LiPF₆ in the concentration of 1 mole/liter into a solvent of1:2 (volume ratio at 20° C.) mixture of ethylene carbonate (EC) anddiethyl carbonate (DEC).
 2. A binder for electrode of lithium ionsecondary battery, which comprises a copolymer obtained bymultistage-polymerizing: a component comprising at least oneethylenically unsaturated monomer whose homopolymerization yields apolymer soluble in N-methylpyrrolidone (NMP) (component (a)); and acomponent comprising at least one ethylenically unsaturated monomerwhose homopolymerization yields a polymer insoluble in NMP (component(b)); which copolymer exhibits a swelling degree of 4 or below, in anelectrolyte obtained by dissolving LiPF₆ in the concentration of 1mole/liter into a solvent of 1:2 (volume ratio at 20° C.) mixture ofethylene carbonate (EC) and diethyl carbonate (DEC).
 3. The binder forelectrode of lithium ion secondary battery according to claim 2, whereinthe multistage polymerization comprises a first polymerization step ofpolymerizing the component (a) and a subsequent second polymerizationstep of adding the component (b) thereto and polymerizing thesecomponents.
 4. The binder for electrode of lithium ion secondary batteryaccording to claim 3, wherein the first polymerization step is a step ofpolymerizing 15 to 80 parts by weight of the component (a) until thepolymerization conversion ratio thereof reaches 60 to 97 weight %, andthe second polymerization step is a step of adding 20 to 85 parts byweight of the component (b) thereto (wherein the amount of all themonomers is 100 parts by weight) and polymerizing the components untilthe polymerization conversion ratio reaches 90 weight % or more of allthe monomers.
 5. The binder for electrode of lithium ion secondarybattery according to claim 2, wherein the multistage polymerizationcomprises a three-stage polymerization process.
 6. The binder forelectrode of lithium ion secondary battery according to claim 5, whereinthe multistage polymerization comprises a first polymerization step ofadding a part of the component (a) and then polymerizing it, asubsequent second polymerization step of adding thereto the component(b) and polymerizing the components, and a subsequent thirdpolymerization step of adding thereto the remaining component (a) andpolymerizing the components.
 7. The binder for electrode of lithium ionsecondary battery according to claim 6, wherein the first polymerizationstep is a step of polymerizing 5 to 50 parts by weight of the component(a) until the polymerization conversion ratio thereof reaches 60 to 97weight %, the second polymerization step is a step of adding 20 to 85parts by weight of the component (b) thereto and polymerizing thecomponents until the polymerization conversion ratio reaches 60 to 97weight % of all the monomers added up to this step, and the thirdpolymerization step is a step of adding 5 to 50 parts by weight of thecomponent (a) thereto (wherein the amount of all the monomers is 100parts by weight) and polymerizing the components until thepolymerization conversion ratio reaches 90 weight % or more of all themonomers.
 8. A slurry composition for electrode of lithium ion secondarybattery, which comprises the binder for electrode of lithium ionsecondary battery as claimed in claim 1, an active material for anelectrode, and an organic liquid medium.
 9. The slurry composition forelectrode of lithium ion secondary battery according to claim 8, whereinthe organic liquid medium is N-methylpyrrolidone.
 10. A productionmethod for a lithium ion secondary battery electrode, wherein the slurrycomposition for electrode of lithium ion secondary battery as claimed inclaim 8 is applied onto a current collector and then dried.
 11. Alithium ion secondary battery electrode, wherein an electrode layercomprising the binder for electrode of lithium ion secondary battery asclaimed in claim 1 and an active material for an electrode is bonded toa current collector.
 12. A lithium ion secondary battery, whichcomprises the electrode as claimed in claim 11.